In a skillet over medium heat, brown ground beef with onion; drain grease. Pour in spaghetti sauce and simmer 5 minutes.
In a 9x13 inch baking dish, spread enough sauce and beef to cover bottom and put a layer of ravioli down end to end. Spread a layer of sauce and cheese just like you would with regular lasagna. Keep layering and finish with sauce on top.
Bake uncovered in a preheated oven for 1 hour. Let sit for 5 minutes before serving.